It's not a "ascii diagram", it's a new syntax for CC3 display property.

Complete example:

  <style type="text/css">
    body {
      height: 100%;
      display: "a   .   b   .   c"  /2em
               ".   .   .   .   ."  /1em
               "d   .   e   .   f"
               ".   .   .   .   ."  /1em
               "g   .   h   .   i"  /2em
               5em 1em  *  1em 10em}
    #logo {position: a}
    #motto {position: b}
    #date {position: c}
    #main {position: e}
    #adv {position: f}
    #copy {position: g}
    #about {position: h}
  </style>